VBScript String Concatenation (And Why It Should Be Avoided Like The Plague)

by Marcus Tucker


On the forums that I participate in (and at work) I often see ASP code making extensive use of string concatenation, and although I want to point out why it's bad and how to improve it, I usually bite my tongue and say nothing because to cover the topic in sufficient depth would take more time than I want to spend! It is disappointing to see that so many developers are unaware of the performance implications of doing so, and in other cases where inefficient techniques are used I would normally provide a link to a relevant article, but so far I haven't yet come across one that does this particular subject justice, which is why I thought that I'd write one myself!

The Problem

Let's start with some simple (but handy) code to manipulate delimited lists:

Function InDelimitedList(byref DelimitedList, byval Value)
  If DelimitedList = "*" Then
    InDelimitedList = True
    InDelimitedList = (Instr("|" & DelimitedList & "|", "|" & Value & "|") > 0)
  End If
End Function

Sub AddToDelimitedList(byref DelimitedList, byval Value)
  If Len(DelimitedList) = 0 Then
    DelimitedList = Value
    DelimitedList = DelimitedList & "|" & Value
  End If
End Sub

Now, as you can see, the AddToDelimitedList() sub uses VBScript string concatenation to tack each value onto the end of the delimited list, and in use this function would be called many times to add items to the list. During development (and perhaps testing), this seems to perform fine, but that's because there's only a few items in the list - only test data is being used. When the code becomes used in a production environment, it could easily cause a major performance because unfortunately VBScript's string concatenation peformance has an N-squared cost, i.e. the time taken to add each additional substring is proportional to the length of the entire string, and thus the total time taken to perform N concatenations follows an exponential curve, which quickly comes to dwarf the execution time for the rest of the code in the script. More on this later.

Of course, the delimited string manipulation code above is merely one example of where you might need to concatenate strings, but it's an interesting example because it's all-to-easy to call code like this from within a loop without realising that it could become a major bottleneck in your application's performance once the usage and/or volume of data increases significantly. Testing during development rarely accurately simulates real-world usage, so performance hits like this may only become apparent once the system is in place... and then it can often be too late to fix! Therefore, you *must* keep an eye out for hidden gotchas like this throughout the development process. So what can you do about it? Read on...


Years ago when I first became aware of the problem I rolled my own VBScript class which used arrays and the Join() function to provide huge efficiency improvements over the native concatenation performance - a technique I had seen in a tutorial somewhere. It was a great improvement, but still had its own problems which were caused by the need to continually ReDim the class's internal array - this became the bottleneck. I later refined this technique by doubling the size of the array with each reallocation, which of course continually halves the frequency of reallocation (at the expense of greater memory use, but that's not usually a problem with web servers), and therefore improves performance quite a bit. This has since always been my preferred technique for concatenating strings together, and the one that I have recommended to others in my capacity as Sitepoint's "ASP Guru", a position I have held for the last couple of years.

However, with the advent of ADO v2.5, some new and interesting objects became available, most significantly the Stream object, which replaces the antiquated (and extremely inefficient) file reading and writing functionality provided by the FSO (FileSystem Object), and adds native binary support (as well as the ability to convert between different data types and character sets). I use the Stream object all the time for manipulation of bulk binary data and text, since it can easily be used as an in-memory buffer (in the same way that a disconnected Recordset object can be used as a simple in-memory flat file database), but I hadn't thought to compare it to my array-based string concatenation class to see if it offered any performance improvements. I therefore wrote a new class to encapsulate the functionality, and a benchmark script to compare the three approaches - native VBScript concatenation, array joining, and the stream object. The results of these tests form the basis for the rest of this article. I should also mention that there are other solutions in the form of 3rd party freeware COM+/ActiveX components (such as Michael Balloni's StrCat.Catter), but as we all know, getting components installed on shared hosting space is nigh on impossible (for good reasons, but that's another story), so it's probably only a viable option for dedicated servers that are yours to do with as you please. And besides, from a portability perspective, it's usually wise to leverage the standard toolset (ADO in this case) as much as possible rather than to use non-standard/custom solutions.


All testing was performed on a spare PC dedicated to running IIS on Windows 2000 Server, and connected to my desktop PC via 100Mb Ethernet. Further details of the machines are irrelevant to these tests since it is the relative performance of figures within each set of results that is important, but be assured that the machine acting as the web server was not modified in any way during the time it took to perform all the tests.

To start with, here are the results of the benchmarks with a substring length of 100 characters, going up to a maximum of 2000 iterations (i.e. for every iteration, a 100-character string was appended to the end of the string being built):

Table A
Substring length: 100
  Time /ms
Iterations   Native   Array   ADO.Stream
200   0.0   0.0   0.0
400   15.6   0.0   0.0
600   15.6   0.0   0.0
800   78.1   0.0   15.6
1000   218.8   0.0   15.6
1200   406.3   0.0   31.3
1400   468.8   15.6   31.3
1600   625.0   15.6   31.3
1800   921.9   15.6   31.3
2000   1,265.6   15.6   31.3
Note that the resolution of the PC clock is +/- approx. 15.6 milliseconds, and therefore timings in the same order of magnitude have a high innacuracy (i.e. accuracy is poor for times below approx. 150ms).

The results in Table A clearly demonstrate that the native VBScript concatenation is extremely inefficient, taking 1.3 seconds to do what the array method can do in a mere 16 milliseconds (approx.)! The stream method takes a similar amount of time as the array method, but due to the innacuracy of the measurements at such short timings, it would be unwise to attempt to infer any more from these results.

Table B
Substring length: 100
  Time /ms
Iterations   Native   Array   ADO.Stream
2000   1,437.5   15.6   46.9
4000   6,796.9   31.3   62.5
8000   -   93.8   125.0
10000   43,515.6   109.4   171.9
20000   -   250.0   328.1
30000   -   359.4   468.8
40000   -   531.3   640.6
50000   -   625.0   796.9
100000   -   1,171.9   1,562.5
Note that because of the long execution times that the native VBScript concatenation incurs, my benchmark script detects step execution times over 2 seconds and skips subsequent timings for that method. Note also that the table does display a couple of timings over 2 seconds and has odd increments of iterations - this is because I have appended results from another run with a different iteration range (but the keeping the substring size the same so that the tests are fair and scientific).

By increasing the iterations, this picture becomes clearer - table B shows that once the number of iterations has been increased, the native method takes a ludicrous 43.5 seconds for 10000 iterations while the array method is 400x quicker, taking a mere 110ms to the same, and the stream method close behind with 172ms.

However, there is another important factor to consider - the size of the substring that is being appended each time. So far all the tests have used a substring 100 characters long - what happens if it's smaller?

Table C
Substring length: 10
  Time /ms
Iterations   Native   Array   ADO.Stream
1000   0.0   0.0   15.6
2000   15.6   15.6   31.3
3000   31.3   15.6   62.5
4000   46.9   31.3   62.5
5000   93.8   46.9   93.8
6000   109.4   46.9   109.4
7000   312.5   62.5   109.4
8000   812.5   78.1   125.0
9000   1,281.3   93.8   156.3
10000   1,312.5   93.8   171.9

Table C shows that the native method performs better with smaller substrings - 10000 iterations in this test took 1.3 seconds, compared to taking 43.5 seconds with a substring of 100 characters (see the previous table). This is because the main cause of the method's performance hit is the overhead of copying around large blocks of memory, so the smaller the substring, the less memory used / copied, and the quicker the time. The array method is still faster than the stream method, but it's now almost 2x as fast! This is most likely to be because the mere act of accessing or calling a method on a COM object incurs overhead, and at small substring sizes these overheads become proportionally significant when compared to the cost of actually storing the 10 character substring once the COM method has been invoked.

Table D
Substring length: 2000
  Time /ms
Iterations   Native   Array   ADO.Stream
1000   8,687.5   46.9   15.6
2000   -   93.8   31.3
3000   -   140.6   46.9
4000   -   171.9   62.5
5000   -   218.8   78.1
6000   -   265.6   93.8
7000   -   296.9   109.4
8000   -   343.8   125.0
9000   -   406.3   140.6
10000   -   437.5   156.3

When the substring length is increased, the tables are turned - table D shows that the stream-based method takes almost a third of the time of the array method to build the string. Again, lest we forget, the array method is approx 180x faster than native, and the stream method is approx 540x faster! Some quick math shows that the final string is 20 million characters long - which to be fair is quite a large volume of data to manipulate - and while it's safe to say that it must occupy a minimum of 9.5MB of memory, since each method stores it in a different way it's impossible to say *exactly* how much actually is used to store it (without extensive further investigation - unicode further complicates matters, for example). Regardless, what *is* clear is that with large substrings the continual ReDim'ing of the array becomes more expensive than simply writing more characters to the end of the in-memory stream. I suspect that the stream object uses a linked list internally to store data across non-contiguous blocks of dynamically allocated memory instead of reallocating memory as a single block each time, since the latter case is of course the Achilles' heel of the native concatenation.


So, we've compared the three approaches, but what have we learned? Well, if nothing else, it should be crystal clear that VBScript string concatenation should be avoided at all costs! That said, it is safe to use native string concatenation when you *know* that only a fixed number of substrings will be concatenated together, such as when you're dynamically constructing a dynamic SQL statement, or assembling a line of HTML to be written directly to an output (file, response, etc). However, as a general rule of thumb, in all other cases, and particularly when the volume of the source data (in data size or number of items) is likely to increase over time, if the number of records which are processed to generate the output are subject to control by the end user (e.g. a selectable search results limit), or you are incrementally generating output which isn't written directly to the output (Response object, disk, etc), you must give due consideration to this issue and use a scalable string building technique.

And which technique should you use? As the results demonstrate, it all depends on exactly *what* you are appending. For short substrings, the array-based methods have the edge, but as you increase the length of the substrings, the stream-based method becomes the winner. Therefore, you should choose your method depending on the nature and volume of the data that you are using. To generalise, since most applications are unlikely to require the frequent appending of thousands of substrings which are each thousands of characters in length, the array method will probably do you just fine in 99% of cases. Incidentally, if you are already using an array-based VBScript concatenation class, check the code to make sure that it is NOT ReDimming the array each time a new substring is added - it's significantly more expensive to use that approach (as I mentioned at the start of the article), so edit it so that it uses the lazy length-doubling method, or simply switch to using my code (see end).

As well as ensuring that you are now fully aware of the performance issues associated with using native string concatenation, hopefully this article has also made you realise that you should try to test your code with real-world data, and to examine alleged performance-enhancing techniques carefully, since whether or not you get a significant gain (or perhaps even a decrease in performance) all depends on the precise circumstances in which you use a given technique - in particular, what data you are passing through it, and how often it gets called.

Finally, the code for the benchmark and concatenation classes is available here: StringConcatenation.zip (2 KB), and you are free to use the code in your own work. I ask only that you leave my credits in place. I hope you've found this interesting reading, and it's always nice to know whether advice has paid off for people, so let me know your feedback, particularly if as a result you've implemented changes which have made a significant difference to the performance of your scripts / web applications.

Additional Information

An example of poorly performing code using the native method - CodingForums

The internal workings of VB/VBScript native string concatenation - Microsoft MSDN

About the Author

From ASP101

Articles originally posted on ASP101.com



  • dev

    Posted by Aaron on 06/09/2015 11:37am

    thank you!! this is awesome

  • lQaBO MyA NqKF

    Posted by rXpqsduOMh on 07/08/2013 08:02am

    buy hydrocodone online hydrocodone sale online no prescription - can order hydrocodone


    Posted by qHYAdzXUJu on 06/24/2013 01:45am

    link viagra online us generic - order viagra ship us

  • clarisonic gain reduced printing is present in australia

    Posted by iouwanzi on 06/05/2013 08:56pm

    [url=http://www.australiaclarisonic.com/]clarisonic mia[/url] Producenter frembringer deres egne beats på nettet åbner ubegrænsede samarbejdsmuligheder med hensyn til designere på et stramt budget. Den særlige udseende tilgængelig er utallige der en bred vifte af friske rekorder på markedet hver dag derude sted. I tilfælde af at du synes at købe overgår, anmeld af din web selskabet samt høre deres varer. Du er i stand til at Google eller måske Ask tilstande såsom “internet-producenter” eller “køb er bedre end online” og masser af troværdige producent internetsider vil helt sikkert producere.En anden tendens jeg opdager i trap rekorder kunne være den store brug af rumklang omkring den primære enhed. Der er typisk en enkelt pind ud værktøj put, der har en velvoksen reverb effekt sige for eksempel en grand violin, guitar, samt synth. I de fleste tilfælde en let melodi, der gentager flere gange samt modulerer op eller ned. Processen mod kaos er meget simpelt, så prøv ikke at under vurdere de grunde, der kan placeres i dette. beats [url=http://www.miaclarisonicaustralia.org/]clarisonic mia[/url] Beats hovedtelefoner har stor detalje, opløsning og billeddannelse. Når du vender kontakten til basforstærkning de er omtrent lige så god på den lave ende som enhver sammenligneligt prissat hovedtelefoner vi har hørt. Når du lytter til kvindelige vokal mere end guitar med støjreducerende på, hovedtelefonerne lyder stadig fuld og tilfredsstillende.Hvis han ikke havde travlt nok hjælpe Intel ud, musik kunstner og producer nu hjælpe miljøet ved at samarbejde med Coca-Cola og Beats med lanceringen af EKOCYCLE. Denne stand-alone initiativ er dedikeret til at fremme genbrug gennem produkter fremstillet i en del af genbrugsmateriale. Dybest set ønsker dig til at tænke genbrug er lige så cool som en iført et par beats hovedtelefoner, eller drikke en dåse cola. [url=http://www.miaclarisonicaustralia.org/]clarisonic mia online[/url] Beats hovedtelefoner har stor detalje, opløsning og billeddannelse. Når du vender kontakten til basforstærkning de er omtrent lige så god på den lave ende som enhver sammenligneligt prissat hovedtelefoner vi har hørt. Når du lytter til kvindelige vokal mere end guitar med støjreducerende på, hovedtelefonerne lyder stadig fuld og tilfredsstillende.Hvis han ikke havde travlt nok hjælpe Intel ud, musik kunstner og producer nu hjælpe miljøet ved at samarbejde med Coca-Cola og Beats med lanceringen af EKOCYCLE. Denne stand-alone initiativ er dedikeret til at fremme genbrug gennem produkter fremstillet i en del af genbrugsmateriale. Dybest set ønsker dig til at tænke genbrug er lige så cool som en iført et par beats hovedtelefoner, eller drikke en dåse cola.

  • beats hovedtelefoner by dr. dre ved, du kan nemt se folk

    Posted by wherewanzi on 06/05/2013 08:50pm

    [url=http://beatsbydrehretelefoner.weebly.com/]beats by dre høretelefoner[/url] If inline controls for calls is a priority, consider checking out the RHA SA950i, which is even more affordable, and also has a more substantial bass response in comparison. The Polk Audio UltraFit 2000 is another budget on-ear option—more affordable than the Zoro with phone controls built into the earpieces, and as a bonus, it’s intended to withstand sweaty workouts. If you have a bit more room in your budget and still want an emphasis on clarity in the mids, and bass that isn’t wildly boosted, the Bowers & Wilkins P3 is a comfortable on-ear option with solid audio performance. [url=http://kbhretelefonerbillig.webstarts.com/]beats høretelefoner[/url] Jeg har efterhånden været indehaver af dette pragteksemplar i knap ét og jeg nyder hvert et øjeblik i selskab med mine Beats. Der er ingen tvivl om, at de har taget verden med storm og de er kommet for at blive. Det kan tage lang tid at finde det rette headset, men med disse Beats by Dr. Dre Studio er jeg tydeligvis ikke gået galt i byen på noget tidspunkt. Hverken i forhold til min stationær, [url=http://beatsbydrehretelefoner.weebly.com/]beats by dre høretelefoner[/url] Introduktion til Beats by Dr. Dre Executive Hovedtelefoner Direktionen repræsenterer en ny karakteristisk, sofistikeret design for Beats By Dr. Dre brand, der fokuserer på håndværk ved hjælp af førsteklasses materialer såsom aluminium, rustfrit stål og læder. Den bløde læder pandebånd og komfortable ørekopper er designet til lange slid og en unik sammenfoldeligt design gør Beats Executive hovedtelefoner nem at bære on-the-go. Den administrerende har overlegen Active Noise Cancellation (ANC) performance, tunet til uafbrudt lytning nydelse ved 35.000 fod, og overalt i mellem og med iOS kompatibel mikrofon kabel, det giver fuld telefon funktionalitet, herunder besvar afslut, opkald venter, switching og stemmekommandoer .

  • Most people gargantuan defeats tender greatest certify along with useful earphone

    Posted by motherdhmm on 06/04/2013 03:51am

    [url=http://blog.cheapbeatsbydre.co.nz/monster-headphone]monster beats headphone[/url] Fake one is tiny. These 2 documents are provided with the real transaction and not with the imitation. The battery deal with of the valid handle, of progression, has the serial number and Dr Dre¡¯s signature.The fake doesn¡¯t have any ( Perfect example inform of fake battery cover not included, valid visualize an empty time where the signature and serial should be). The differences fish tale in the constituents of the discrimination cup and the aforementioned battery covers.The battery arraign for light on the real handle also seems to be a a tad brighter [url=http://blog.cheapbeatsbydre.co.nz/]beats by dre[/url] Metal lamination with B logo ¨C Genuine is more non twinkling while pretend metal logo looks shimmering! Impostor has darker blacks and whiter whites! Authentic box has nicer NATURAL lighting.we know that the mask covers are almost same except for the treatment of the lighting on the headphones.The lighting on the honest hide-out keep an unconcealed natural half dark half grayish tint to it while the fake has a more moral peg sombre feather of lighting. [url=http://www.headphonescheaponlineaustralia.com/beats-pro-c-66_68_75.html]beats Pro[/url] But at the having said that for the present, it also has traditional shortcomings. The origin outline: The Beats Unaccompanied about Dr. Dre Headphones from Fiend proposal a torpedo, travel-friendly pattern that¡¯s positive to diminish heads, but the audio quality fails to survive up to the price tag.The Beats Solo headphones may not be carefree for everybody under the sun, audio is tone down complete, and they split a legitimate amount of sound.

  • Az6 sac pliable longchamp 32664

    Posted by jelsnitmete on 05/14/2013 01:59am

    All in all, Web Analytics in SharePoint 2010 will help you better understand what users want from your site, what are the popular content on your site, etc.All those songs and pictures are sitting on your computer making it go slower.People are clinically overoptimistic, for instance, assigning zero probability to events that are merely unlikely (such as a massive iceberg in the path of a really big ship). sac à main longchamp ? ghd straighteners Powware. ルイヴィトン 財布 Generally speaking, if a website's goal is to monetize content that is not original or high quality, then it is likely that the site has now been targeted by Google. [url=http://sacslvonline.webnode.fr]sacslvonline.webnode.fr[/url] Then press it down on the workpiece and tilt is slightly as you scrape, see Fig.sac longchamps Livingstone learned they presumed that Africans taken by whites were fattened and then consumed. mulberry bags outlet The slide and settle is a quick movement the whole thing's over in 1015 seconds but it's impressively accurate.Each section can also be paired with a color along with the direction of the five elements are mapped out as well. When he dropped the clothespin or tipped the books over, the toddlers would race to pick up the clothespin and hand it back, or restack the books.Yet another tool targeted to the consumer is the free sample.Basically, the international famous brands 3M, ELO, GVision, etc. ルイヴィトン バッグ Wherever your job site is we can deliver what you need, when you need it.

  • http://www.oakleysunglassesoutc.com/ mgjmly

    Posted by http://www.oakleysunglassesoutc.com/ Suttonsqi on 03/28/2013 09:17am

    ghd straightener,Not worried about international opinion Tan Yan, this time can be a strong international coalition organizations to interfere in China's ground military operations, this incident behind China and Japan, the United Kingdom rivalry the British reaction fierce that Tan Yan shall Do not consider the next in the softening in some water mill tactics taken by the British negotiations on the Concession and Hong Kong colonial problem - recently negotiated very smoothly. Table a series of speeches Chamberlain in London, the United Kingdom, the Brits seem to want to fight for the self-esteem of the ghd hair straightener and moving war, which makes originally weakening ground at the negotiating table, Zhu Jordan seems to have played a needle with blood began tough up. The afternoon back to Zhongnanhai, Tan Yan went to the Ministry of Foreign Affairs - ghd sale,hairstraighteneraul.ghd hair straightener,com/" title="ghd"ghd has invited countries to minister to the Sea of ​​Japan to China sank two armored cruiser event in Japan talks, and subsequently held a news Hutchison will also.

  • ghd australia njflzg

    Posted by Suttoncmk on 02/03/2013 09:19pm

    8aFgv ugg hXwg gIpc nike 8dXfm toms outlet 2mLmx hollister uk 5cDir ugg 0eCkm longchamp pas cher 9xZfl louis vuitton outlet 5vJct michael kors outlet 2iLtz christian louboutin 6hNra Scott Tolzien Jersey 4hVmm 0qZkq 5jIrx ghd 3nIeu ugg sale

  • ghd australia watamf

    Posted by Mandylec on 02/03/2013 03:45pm

    5yAhj ugg nXjo ¥È¥ê©`¥Ð©`¥Á mKbe nike sko p? nett 1uHsy toms shoes on sale 4gLaa hollister uk sale 3nRmo bottes ugg 1gAzw longchamp pas cher 2eMov louis vuitton purses 5xYry Michael Kors 7mFof christian louboutin norge 8xKqe colin kaepernick jerseys 2cQxz 0mIab GHD Hair Straightener 2eTxk ghd 0lGid cheap uggs

  • Loading, Please Wait ...

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • As all sorts of data becomes available for storage, analysis and retrieval - so called 'Big Data' - there are potentially huge benefits, but equally huge challenges...
  • The agile organization needs knowledge to act on, quickly and effectively. Though many organizations are clamouring for "Big Data", not nearly as many know what to do with it...
  • Cloud-based integration solutions can be confusing. Adding to the confusion are the multiple ways IT departments can deliver such integration...

Most Popular Programming Stories

More for Developers

RSS Feeds

Thanks for your registration, follow us on our social networks to keep up-to-date